1. Field of the Invention
The present invention relates to valve assemblies which control hydraulically powered machinery; and more particularly to pressure compensated valves wherein a fixed differential pressure is to be maintained to achieve a uniform flow rate.
2. Description of the Related Art
Agricultural, construction and industrial machinery have components that are moved by hydraulic actuators, such as cylinder and piston arrangements. Application of hydraulic fluid to the hydraulic actuator is often controlled by a valve with spool that is moved by a manually operated lever. Solenoid operated spools also are available. Movement of the spool into various positions within a valve body proportionally varies the flow of pressurized fluid from a pump to one chamber of the cylinder and controls fluid draining from another cylinder chamber. Typically a plurality of valves for operating different hydraulic actuators were combined side by side in sections of a valve assembly.
The speed of a hydraulically driven component on the machine depends upon the cross-sectional areas of control orifices in the spool valve and the pressure drop across those orifices. To facilitate control, pressure compensating hydraulic control systems have been designed to set and maintain the pressure drop. These previous control systems include load sense lines which transmit the pressure at the valve workports to the input of a variable displacement hydraulic pump which supplies pressurized hydraulic fluid in the system. The resulting self-adjustment of the pump output provides an approximately constant pressure drop across a control orifice, the cross-sectional area of which is varied by the machine operator. This facilitates control because, with the pressure drop held constant, the speed of the machine component is determined only by the cross-sectional area of an operator variable metering orifice.
One such prior system is disclosed in U.S. Pat. No. 5,579,642 entitled “Pressure Compensating Hydraulic Control System”. That system utilized a chain of shuttle valves to sense the pressure at every powered workport of each valve section and to choose the highest of those workport pressures. The chosen workport pressure of that chain was applied to an isolator valve which connected the control input of the pump to either the pump output or to the system tank depending upon that workport pressure. The isolator valve was contained in a separate, special end section of the valve assembly.
The control pressure applied to the pump's control input also was applied to a separate pressure compensating valve in each valve section. In response to the control pressure, the pressure compensating valve created a substantially fixed differential pressure across the spool by controlling the workport pressure after the fluid flowed through the valve spool.
U.S. Pat. No. 5,892,362 entitled “Hydraulic Control Valve System With Non-Shuttle Pressure Compensator” eliminated the separate isolator valve. In this apparatus, each pressure compensating valve has a poppet and a valve element both of which slide reciprocally in a bore of the valve section. The poppet functions as the prior pressure compensating valve. The valve elements in all the valve sections cooperatively applied the greatest workport pressure to the pump control input. Each valve element also acted on the adjacent poppet in response to that control pressure.
However, that previous valve assembly required two active components in each section's pressure compensating valve. It is desirable to simplify the structure of the pressure compensating mechanism further and reduce its manufacturing complexity.